The mechanical determinants of blood flow will be determined in whole lungs and in small subunits of lung using isolated blood-perfused lungs. A limited species comparison will be made of blood flow in the whole lungs of rabbits, ferrets and dogs to evaluate these species as an appropriate animal model. To study blood flow in small lung units, radiolabeled microspheres will be injected into the lungs, and distribution of blood flow will be quantified by counting radioactive decay from pieces of lung sampled with a cork borer from slices of the dried lung tissue. Effects of changing vascular pressures and lung volume will be separately evaluated in groups of dogs. In addition, the hypothesis that anatomical position per se is an important determinant of local perfusion will be tested by correlating local flow to distance between a sampling site and a major lobar blood vessel or the lung surface. These results will enhance our understanding of the factors that influence the distribution of blood flow in normal, nonedematous lungs. Knowledge of the distribution of blood flow is important, since the matching of blood flow to ventilation is crucial for optimal gas exchange.